Polymer light emitting electrochemical cells (LEC), which are useful in, among other applications, organic electro-luminescent flat displays and large area light sources, have the function of providing light emission upon an applied voltage without the need for a sensitive low work function metal cathode, which is needed in normal polymer light emitting diodes (PLED). In an LEC a solid electrolyte provides, upon the application of a voltage, the ion gradients needed for the injection of electrons from the cathode also in cases where the cathode is not a low work function electrode. A problem with an LEC is that ion travel in the electrolyte makes the response slow. G. Yu et al have in their article “Polymer Light-Emitting Electrochemical Cells with Frozen p-i-n Junction at Room Temperature”, which was published in Advanced Materials, 1998, 10, No. 5, page 385-388, proposed a solution to this slow response by providing an LEC having so called “frozen p-i-n junctions” and in practical operation working as a PLED. The frozen junctions were obtained by providing an electroluminescent polymer, mixing it with an electrolyte and sandwiching it between two electrodes to obtain a LEC. The LEC was heated to about 60-80° C. and then a voltage of about 3-4 V was applied. After the ion gradients had been generated the LEC was cooled to room temperature. The ion gradient became “frozen”, i.e. the ions did not return to their normal position since their mobility was too low in the polymer in question at room temperature.
A disadvantage of the LEC according to G. Yu et al is that it is complicated to manufacture. Further it is sensitive to high temperatures, since an increased temperature would mean that the ions would start travelling again and would thus destroy the “frozen” ion gradients.